1. Field of the Invention
The present invention relates to a gas feeder for feeding a fixed mass flow of a source gas used, e.g., in a CVD (chemical vapor deposition) process.
2. Description of the Related Art
In a process of growing a thin single crystal layer, e.g., of a semiconductor, the CVD process is employed. In the CVD process, a source gas is fed as a raw material for a single crystal layer deposited by a chemical reaction on the surface of a substrate. In comparison with other processes, the CVD process is characterized in that it is easy to add an impurity to the single crystal layer during formation of the layer, a less amount of an unnecessary impurity contaminates the layer and a step coverage characteristic is good. Therefore the CVD process is often employed, especially, in the production of semiconductor wafers. However, the CVD process has a technological disadvantage that a predetermined thickness of a layer cannot be obtained unless the flow of the source gas is accurately controlled during formation of the layer.
FIG. 2 illustrates a prior art gas feeder 21 for use in the CVD process. The carrier gas from a carrier gas feeder (not shown) passes through an input gas pipe 22, a mass flow sensor 23, a flow control valve 24 and a ratio detector 25 in the order mention into a source liquid 27 which is held in an agitator or bubbler 26. A mixture of a source gas and the carrier gas is prepared by mixing the carrier gas with the source liquid 27 and vaporizing same in the agitator 26 and goes out through an output gas pipe 28 equipped to the agitator 26. The gas mixture passes through the output gas pipe 28 and the ratio detector 25 to a CVD furnace (not shown).
A process computer system 29 regulates volumetric flow (i.e. volumetric flow per unit time) of the source gas of the gas mixture to a fixed value. Operator determines the flow of the source gas using a potentiometer 30. The mass flow sensor 23 senses the flow of the carrier gas passing through the input gas pipe 22 and sends a flow output signal to an arithmetic circuit or CPU 31. The ratio detector 25 detects concentration of the gas mixture passed through the output gas pipe 28 and sends a concentration output signal to the CPU 31. The CPU 31 computes the flow of the source gas in response to the flow output signal from the mass flow sensor 23 and to the concentration output signal from the ratio detector 25. A control circuit 32 receives a set-flow signal from the potentiometer 30 and a computation output signal from the CPU 31 and outputs a control signal to the flow control valve 24 in response to a difference from the set-flow signal and the computation output signal. The control circuit 32 controls the degree of opening of the flow control valve 24 in response to the difference from the set-flow signal and the computation output signal to control volumetric flow of the source gas fed to the CVD furnace so as to fix mass flow of source gas.
The gas feeder 21 controls volumetric flow of the source gas instead of mass flow of the source gas from the following causes: In essence, it would be preferable to directly control mass flow itself of the source gas fed to the CVD furnace. However, it is difficult to directly control mass flow itself of the source gas because of the state of being gas. Therefore the gas feeder 21 controls volumetric flow of the carrier gas to fix mass flow of source gas, assuming that mass flow of the source gas is approximate to volumetric flow thereof.
As shown in FIG. 2, a display 33 indicates volumetric flow of the carrier gas, concentration of the gas mixture and volumetric flow of source gas.
The prior art gas feeder 21 has the following drawback: Since the flow control valve 24 provided on the input side of the agitator 26 directly controls volumetric flow of the carrier gas to indirectly control volumetric flow of source gas, a time period is long in which the carrier gas passes through the input gas pipe 22 between the flow control valve 24 and the agitator 26 and the gas mixture passes through the output gas pipe 28 between the agitator 26 and the ratio detector 25. Therefore change of volumetric flow of the source gas in response to change of volumetric flow of the carrier gas requires a prolonged time. This causes the response time of a feedback loop of the mass flow sensor 23, the ratio detector 25, the flow control valve 24, the agitator 26 and the process computer system 29 to be prolonged, so that control of mass flow of the source gas is unstable and mass flow of the source gas fed to the CVD furnace highly is changed. Therefore it is difficult to accurately control the thickness of a layer deposited on a substrate in the CVD furnace.
In a method of controlling volumetric flow of only source gas, the pressure, the temperature and the volume of residual source liquid in the agitator during agitation, cause an actual mass flow of the source gas to differ from mass flow of the source gas estimated from volumetric flow of source gas. Therefore it is difficult to accurately control mass flow of the source gas fed to the CVD furnace.
A control method in which a gravimeter or the like measures the weight of source liquid to determine the actual mass flow of the source gas fed to the CVD furnace requires a long time to a time at which actual mass flow of the source gas reaches a predetermined value. Thus this method cannot accurately control mass flow of source gas.